1. Field of the Invention
The present invention relates to an ink jet recording sheet and a process for producing the same. More particularly, the present invention relates to an ink jet recording sheet for recording images and letters formed by jetting an aqueous ink containing a water-soluble dye, and a process for industrially producing the same.
2. Description of Related Art
It is known that a recording sheet for forming a hard copy having a high quality, in accordance with an ink jet recording method, is provided by coating a surface of a substrate sheet, for example, a paper sheet, with a coating composition comprising a pigment and a resinous binder to form a coating layer which allows small drops of an aqueous ink jetted on the surface of the coating layer to rapidly penetrate into the coating layer in a direction at right angles to the surface of the coating layer without spreading on the surface of the coating layer, to form clear images consisting of a number of dots on the coating layer surface.
For example, Japanese Unexamined Patent Publication (Kokai) No. 55-51583 (1980) discloses an ink jet recording sheet having a coating layer formed on a substrate sheet surface and comprising non-gelatinous silica particles having a size of from 0.1 to 10 .mu.m and a resinous binder.
Also, Japanese Unexamined Patent Publication (Kokai) No. 56-145856 (19,81) discloses an ink jet recording sheet having an ink-receiving layer formed on a substrate sheet and comprising a mixture of fine silicic acid particles and a resinous binder soluble in a non-aqueous solvent or a mixture of fine silicic acid particles, other inorganic pigment particles, and a resinous binder soluble in a non-aqueous solvent.
The above-mentioned conventional ink jet recording sheets are disadvantageous in that the coating layer has an unsatisfactory mechanical strength and, therefore, the surface strength of the coating layer does not reach a required level, and thus the color-developing property for the aqueous ink and the resolving power of the coating layer are not always satisfactory.
Where the ink-absorbing property, resolving power and color-developing properties of the conventional coating layer must be enhanced, the content of the ink-absorbing pigment, such as silica, in the coating layer must be increased, and the amount of the coating layer must be also increased. The above-mentioned requirements make it difficult to enhance the mechanical strength of the coating layer.
Further, Japanese Unexamined Patent Publication (Kokai) No. 62-83178 (1987) discloses an ink jet recording sheet having a coating layer formed on a substrate sheet surface, comprising, as a principal component, a mixture of fine silicic acid particles and a cationic emulsion polymer and exhibiting improved mechanical strength and a high quality image-forming property. Nevertheless, the conventional recording sheet having the above-mentioned, improved coating layer still does not always absorb the aqueous ink at satisfactory high speed, nor dies it always have a high surface strength of the coating layer. Further, the images or letters formed by the aqueous ink are disadvantageous in that they have a poor resistance to water.
To provide water-resistant images or letters on a recording sheet, Japanese Examined Patent Publication (Kokoku) No. 62-11678 (1987) discloses a coating layer containing, as a resinous binder, a water-insoluble copolymer having 10 to 90 molar % of ethylenically unsaturated cationic comonomer. Also, Japanese Unexamined Patent Publication (Kokai) No. 56-84992 (1981) discloses a recording sheet having a coating layer which contains a water-soluble, polycationic, electrolytic polymer. However, the above-mentioned conventional coating layer containing the cationic, water-insoluble polymer has an unsatisfactory ink-absorbing speed and resolving power. Also, the water resistance of the resultant images on the coating layer is poor, the cationic polymer in the coating layer exhibits a poor resistance to weather, and the color of the coating layer is discolored yellow with a lapse of time.
The water resistance of images can be enhanced only by using a water-soluble cationic polymeric compound. This water-soluble cationic polymeric compound can be utilized together with the water-insoluble cationic copolymer containing, as a copolymerization component, a fatty acid vinyl ester comonomer, to enhance the water resistance of images, the ink-absorbing property, and the resolving property of the coating layer.
Japanese Unexamined Patent Publication (Kokai) No. 62-83178 (1987) and Japanese Examined Patent Publication (Kokoku) No. 62-11678 (1987) disclose specific water-insoluble, cationic polymers for forming the coating layer of the recording sheet, but since these cationic polymers, which are usually in the form of an aqueous emulsion, have a high cation density when used in the preparation of a coating liquid or in a produce for the coating thereof, and the pigment particles, for example, silica particles, have a negative charge, it is practically difficult to provide a coating liquid by evenly dissolving the cationic polymer in water in the presence of the pigment and evenly dispersing the pigment particles in the cationic polymer solution. Also, in the preparation of the coating liquid, sometimes the viscosity of the resultant coating liquid is undesirably increased or the coating liquid is coagulated, and thus the coating and drying procedures become unstable.
Accordingly, a strong demand has arisen for a new type of ink jet recording sheet which can absorb ink in a satisfactory amount at a high speed and form clear color images of ink dots with a high resolving power, and exhibit an enhanced surface strength and satisfactory practical handling properties, for example, processing, printing, cutting and writing properties, and for a new type of process for producing the ink jet recording sheet by using a coating liquid having an improved coating property and stability.